Leaky alchemy jug.
Whenever you use this jug roll a d100. This is how many percent of your choosen liquid are replaced by mayonnaise.

Bag of holding with a preexisting hoarding problem

Can’t-trip - only works when you don’t want it to

Book of spells, but you have to spell out letter by letter each time you cast

A potion of True Healing… heals 1d8+2 damage, recipient MUST truthfully answer the next question they are asked. Sell the characters 6, but don’t tell them about the truth serum. Let them figure it out on their own.

Boots of Elvenkind… except Elves can hear you.

A bag of holding that contains infinite clowns. Every time it is opened, 1d4 clowns come out. The clowns are useless in combat and attempt to distract, annoy and mock the holder. While this could be used as a distraction, the clowns will follow the holder, drawing attention to them. You could create a table for what kind of clowns you get (mime clowns, pie throwing clowns, balloon animal clowns, magician clowns, etc). The clowns will wander off after 1d6 minutes. Where the clowns go and what they are (Illusions? Demons?) is unknown.

Scroll of summon wisp.

When used nothing appears, but you gain a speech impediment for 1d6 days.

There’s so much role playing potential in the ability to create a giant mob of clowns at will by repeatedly opening and closing the bag. You almost don’t need anything else!

Spawn them as a distraction!

Use them to hide!

Plug any entry or hallway at will!

Build yourself a mountain of clowns to scale any wall!

Never starve again with their endless supply of pies! (Eaten fresh off your face.)

Use their weight to bring down any air-/ship!

Air drop them on your enemies! (Assuming they have a weight and are bound by gravity, they do damage - all you need is a bit of levitation, a tower, airship or a ceiling to hang from.)

Just crush your entire party by spawning hundreds of them in a closed room!

The possibilities are truly endless.

Caltrops of Seeking. Drawn to metal and increase chance to damage/hit metal boots.

They stick together though and must be manually placed one by one over an extended period of time.

Just crush your entire party by spawnibg hundreds of them in a closed room!

Relevant (semi-NSFL) scene from Invincible season 3.

Owo what a tewwible cuwse to put on youwselwf…

Flawed bag of holding - it’s a bag of holding, but the dimensions inside are normal bag sized.

Movable rod - an immovable rod, but it can only resist up to 10lbs

Bed of comfort - it’s comfortable, just not the way you were hoping.

Vorple blade - it’s exactly what it is, but it’s too dull to cut and sharpening it would damage the enchantment.

As a diabolical GM, I can think of so many ways to make these strategies backfire.

I have two I will be using in my next campaign:

Ring of attunement: Provides 1 extra attunement slot. (Requires attunement)

Event Staff: This staff allows the wielder to gain unquestioned entry into any “employees only” areas or zones otherwise off-limits to the public. Anyone (including actual staff or other officials) who sees the wielder in one of these areas will assume they are a known employee or other official who is granted special access to the area. Unfortunately, they will all also view the wielder as the least competent and least trustworthy employee or official with the organization. Any actions taken in the area are likely to be closely watched and highly scrutinized by any observer who would know better.

ETA: One from the current campaign in which I am a player character. Our DM thought of this one:

Bullet of Healing:

This magical bullet can be loaded into any firearm. Whomever is shot by this bullet first receives 1d6 piercing damage followed by 1d10 healing. If the initial damage causes recipient’s HP to fall below 0 before the bullet’s healing effects begin, they will fall unconscious and will not gain any healing effect from the bullet. Instead, one death save is automatically passed.

Wand of wishes. It’s great, but it will respond to any speaker in the vicinity

Bag of folding.

A bag of holding except anything stored in it comes out folded in half.

Cloak of Levitation, but it just lets you float a foot off the ground without moving.

Hulk Strength, but dumber than Hulk. You smash anything in the area, including your supplies/allies/horse…

Cap of Invisibility, but you can’t see anything either.

That’s half the fun! Sometimes, the true clown you spawn is yourself.

Jesus…

Sir Mix-a-Lot (unrelated) is a traveling potion salesman who shows up for my party at suspiciously specific times, and generally has discounted potions specifically tailored for whatever they happen to be doing at the time. For example, if they need to be really strong, he’ll have a bottle of Sir Flex-a-Lot’s Magical Muscle Maximizer, which does increase the strength of one’s muscles, but not of their bones or connective tissue (it was designed to be used only in bodybuilding competitions), so whenever the drinker does a STR check, they must also make a CON saving throw to avoid breaking a bone or tendon. Need to decipher an ancient text? Try Sir Scripts-a-Lot’s Polyglottal-in-a-bottle, which will let you read unknown languages, but also comprehend all unknown languages, even those of the plants and animals around you, making it very difficult to concentrate on any one thing. (inspired by https://www.smbc-comics.com/comic/springtime)

Sword of ghostly might: One owned by a powerful warrior who came back as a vengeful spirit. Neglects to mention that the sword is also a ghost, and therefore can only deal damage to spirits.

I can’t see any of these working as intended. Clowns don’t subscribe to reality

Spawn them as a distraction!

Some of them cause a big distraction that accidentally points directly towards those you don’t want to be seen.

Use them to hide!

One of them will look giant and big to hide you while the others honk and gesture/point behind, clearly showing where you are.

Build yourself a mountain of clowns to scale any wall!

Crabs in a bucket. None will let you climb. You must stay to hear their jokes…

Never starve again with their endless supply of pies!

Shaving cream pies. Ain’t nobody got time to bake 30 coconut creams

Use their weight to bring down any air-/ship

They all blow up helium balloons to help it float. Unless you want it to float in which case their balloons turn into bowling balls at the last second with a big shrug.

Air drop them on their enemies!

See balloons

Just crush your entire party by spawning hundreds of them in a closed room!

Clown car logic. You’re all “crushed” but it’s just extremely difficult terrain.

I have a spreadsheet full of these somewhere. The 2 that my players got that I remember them using are:

Emperors armor: +3 full plate armor with no strength or armor proficiency requirement. When you look at yourself or in a mirror you see yourself in full plate armor, but to everyone else you are naked and if anyone tells you that you are naked the armor and all your possessions cease to exist. My player made it the shopping district in the heart of the city before someone told him.

Staff of disintegrating: when activated, it disintegrates. The player that got this one saved it for a boss fight. He found it hilarious though.